According to the prior art, the hydraulic adjustment of a variable displacement pump is controlled by a load responsive signal, i.e., the load pressure of the highest loaded consumer, in order to increase the delivery rate of the pump. It is further known to provide a delivery rate control valve for the hydraulic adjustment, wherein the piston of the valve is in communication via a signal conduit and a shuttle valve row to all consumers in order to supply the respective highest load pressure of a consumer via the signal conduit to the control valve.
In a situation wherein the variable displacement pump is delivering fluid at its maximum delivery rate, yet an increase in the delivery volume for a consumer is nevertheless required, which requirement is communicated via the signal conduit, the hydraulic system falls into a mode of undersupply. To remedy this condition, it is known to reduce the flow rate through the consumers; for example, by reducing the flow rate through the consumers by means of respective pressure compensation valves which are correlated to the consumers (German Patent Application P 35 32 816.9). To reduce the flow rate through the consumers, it is further known (P 36 44 745.5) to provide the respective pressure compensation valves with a supplementary control land. When the pressure compensation valve is in its open end position, the supplementary control land is operable to permit a reduction in the pilot control pressure for the hydraulically controlled directional valves. Accordingly, when such a pressure compensation valve locates at such an end position, the flow rate through all consumers is reduced. Thus, according to the prior art, the control signals for increasing the delivery rate of a hydraulic pump, or for reducing the fluid flow rate through the consumers, was obtained from the highest loaded (i.e., load leading) consumer, once the pressure compensation valve of such consumer reached the end of the control range.